It is generally recognized that the production of noxious oxides of nitrogen (NOx), which pollute the atmosphere, are undesirable and, in many cases, are controlled by limits established by local, state and federal governmental regulations. The formation of NOx constituents in the exhaust gas products of an internal combustion engine must therefore be eliminated, minimized, or at least maintained below some threshold limit or level.
It is generally understood that the presence of NOx in the exhaust of internal combustion engines is determined by combustion temperature and pressure as well as by the air/fuel ratio (lambda). An increase in combustion temperature causes an increase in the amount of NOx present in the engine exhaust. Therefore, it is desirable to control the combustion temperature in order to limit the amount of NOx present in the exhaust of an internal combustion engine.
One method suggested by the prior art for limiting or controlling the combustion temperature has been to recirculate a portion of the exhaust gas back to the engine air intake. It was reasoned in these early methods that since the exhaust gas is low in oxygen, this will result in a dilute combustion mixture which will burn at a lower temperature. The lower combustion temperature, it was reasoned, would, in turn, reduce the amounts of NOx produced during combustion.
Also, it had, until recently, been common practice to run an internal combustion engine at or near an ignition timing that produces peak combustion pressures, which maximize combustion efficiency. However, unacceptably high levels of NOx may be produced in the combustion chambers when the engine operates at or near such conditions. Therefore, in order to inhibit the formation and emission of NOx, it is necessary to limit the peak combustion pressure to a threshold value.
One technique suggested by the prior art for limiting combustion pressure involves the recirculation of exhaust gases through the induction passage of the combustion chamber since it is well-known that an increase in recirculation of exhaust gases will reduce peak combustion pressure, and thus, the attendant levels of undesirable NOx.
Therefore, it has become generally well-known that the formation of undesirable oxides of nitrogen may be reduced by recirculating a portion of the exhaust gas back to the engine air/fuel intake passage so as to dilute the incoming air/fuel mixture with inert H2O, and CO2. The molar specific heat of these gases, and especially of CO2, absorbs substantial thermal energy so as to lower peak cycle temperatures and/or pressures to levels conducive to reducing NOx formation.
While NOx formation is known to decrease as the exhaust gas recirculation (EGR) flow increases to where it represents a threshold percentage of the exhaust gas constituents, it is also known that this is accompanied by a deterioration in engine performance including, but not limited to, an increase in engine roughness with increasing EGR. Therefore, one factor limiting the magnitude of EGR is the magnitude of EGR-induced performance deterioration or roughness that can be tolerated before vehicle drivability becomes unacceptable.
Accordingly, various systems have been suggested to control the amount of exhaust gas flowing through the system, such as those disclosed in U.S. Pat. No. 5,333,456 to Bollinger and U.S. Pat. No. 6,502,397 to Lundqvist. These systems uses valves or sleeves to partially block the flow of exhaust gas before it mixes with inlet air, thereby controlling the amount of exhaust gas versus inlet air existing in the resultant mixture.
However, these arrangements result in a number of disadvantages. One problem with these devices is that they require extra components in addition to the standard piping for the inlet air and exhaust gas flows that, in addition to increasing the cost and difficulty of manufacture and assembly, requires additional space in the vehicle. Moreover, the specific components employed and the arrangement thereof do not facilitate as efficient of a mixing of the two gas flows as is possible. Additionally, the particular arrangements of these parts result in systems that are less accurate than desirable in obtaining both precise amounts of both gas flows and a precise ratio between the two different flows. Finally, such systems are unable to completely terminate the flow for whichever of the gas flows it may be desired to do so.
What is desired, therefore, is a system for controlling the mixture of inlet air and recirculating exhaust gas that optimizes the mixing efficiency of the two flows. What is further desired is a system for controlling the mixture of inlet air and recirculating exhaust gas that does not require additional components connecting to the existing piping requiring excess additional cost and space. What is also desired is a system for controlling the mixture of inlet air and recirculating exhaust gas that can precisely control the ratio of inlet air versus exhaust gas, including the complete termination of whichever gas flow may be required.